There are several types of retroreflective sheeting: exposed lens (as taught in U.S. Pat. Nos. 2,326,634 and 2,354,018), embedded lens (see U.S. Pat. No. 2,407,680; 4,367,920 and 4,505,967) and encapsulated lens (see U.S. Pat. No. 4,025,159). The drawings of the aforementioned patents illustrate the various sheeting types and are incorporated by reference herein. Retroreflective sheeting, in general, comprises a multiplicity of lens elements and a reflector located behind the lens elements. The lens elements may be, for example, glass or ceramic microspheres in a polymeric base sheet. The reflector may be: a layer of reflective metal (e.g. aluminum which has been vapor deposited on the lens elements or at some location behind the lens elements); flakes of reflective metal embedded in a binder layer; or a dielectric reflector as taught in U.S. Pat. No. 3,700,305.
Exposed lens retroreflective sheeting generally comprises a polymeric binder film in which is partially embedded a multiplicity of lens elements (e.g. glass microspheres), and a reflecting means disposed on the back (embedded) side of the lens elements.
Embedded or enclosed lens sheeting may be comprised of: a back reflector; an overlying transparent spacing layer; a layer of small lens elements embedded in the spacing layer and in optical connection with the reflector and spaced from it so as to place the reflector at the approximate focal point of the lens elements; a transparent covering layer conforming to the front extremities of the lens elements and having a flat front face; and an optional transparent top layer for improving weatherability.
Encapsulated lens retroreflective sheeting may, for example, be comprised of a plurality of glass microspheres having their back surfaces partially embedded in a transparent polymeric layer with a reflective layer adjacent to and disposed behind the embedded back surfaces of the microspheres and another transparent polymeric layer over the front of the microspheres and bonded to the first transparent polymeric layer in such a way as to result in air cells in front of the microspheres.
Retroreflective sheeting has many applications. Exposed lens sheeting with a dielectric reflector is used as a security film placed on top of documents such as driver's licenses, passports, and automobile titles. They can bear identifying indicia such as a state seal or watermark which can be seen at certain angles, but which do not interfere with the legibility of the document. Exposed lens sheeting is also useful as a safety item on garments to make them reflective at night, such as policeman's uniforms, fireman's coats and jogging outfits.
Both enclosed and encapsulated lens sheeting are useful in applications where they will be exposed to rain because the clear plastic layer which lies over the microspheres or other lens elements helps to prevent water films from adversely affecting the optics of the lenses. Thus, embedded lens sheeting is used in license plates and validation stickers for automobiles, and both encapsulated lens and enclosed lens sheeting have been used in highway signs and reflectors.
In use on highway signs and reflectors (e.g. pavement delineators placed on the road) it is desirable for sheeting to be soil resistant and dew repellent. The accumulation of dirt and oil from the air and road surface onto retroreflective sheeting is detrimental to the retention of its retroreflective brightness. Thus, the term "soil repellent" or "soil resistant" means reducing adherence of soil particles to the reflector surface.
It is well known that droplets of water (e.g., dew) can condense from a humid atmosphere onto a cool surface. Such condensation formed upon the surface of retroreflective sheeting impairs the transmission of light through it. The term "dew modifier" as applied to retroreflective sheeting means a material which is able to avoid a substantial decrease of retroreflective brightness due to formation of water droplets on the surface, for example by causing the dew or water to wet the surface and form a sheet or layer of water rather than droplets.
It is also desirable for the sheeting to be abrasion resistant, especially when it is to be used on the road surface. Retroreflective sheeting is applied to road surfaces in the form of pavement marking tape and as the reflective element in pavement delineators or pavement markers. In these applications, the sheeting is actually contacted by vehicle tires as they traverse the pavement markers or lane marking tape. The force of the tires, combined with any road grit, can abrade the reflective sheeting.
The use of silica coatings to modify the surface of light transmissive materials is known (e.g. antireflective coatings). However, the desired combination of soil repellency and dew modification have not yet been obtained with a silica coating that is able to tenaciously adhere to retroreflective sheeting.